Biology Reference
In-Depth Information
elephant grasses. No matter how one defines evolutionary success—diversity of species, per-
sistence over time, range size, biomass, or feeding niches occupied—rhinos dominated the
early epochs when large mammals flourished, in the late Eocene and the Oligocene. On the
basis of their numerical dominance, their biomass within an ecosystem, and the number of
diverse, wide-ranging, but now extinct species that once flourished, we can safely say that
many ancient rhinos, when compared with other mammalian browsers and grazers, were eco-
logical commoners: widespread and abundant.
So what caused the eventual decline of this group? While large body size does not cause
rarity, there is an important implication to it that can be drawn from studies of the mass ex-
tinction of large mammals during the Pleistocene epoch. The once abundant large plant-eat-
ing mammals—the mammoths, mastodons, and their kind—were especially vulnerable to en-
vironmental changes and relatively easy to hunt. Early hunting hit large animals especially
hard. A major reason is that large animals, especially herbivores, inevitably have very low
reproductive rates. Their persistence depends on high adult survival rates. Thus, even a slight
increase in death rate can bring their numbers down rapidly. When humans, whether early
hunters or latter-day poachers, take females from the population time after time, crash is not
far behind.
In North America, almost the entire rhinoceros fauna was eliminated between 2.5 and 5
million years ago, and in Eurasia only two lineages survived. One of these gave rise to a form
quite similar to the Sumatran rhinoceros of today. Perhaps the most famous member of this
line was the woolly rhinoceros (
Coelodonta antiquitatis
), which appeared first in China and
moved westward into Europe. This charismatic thick-coated, big-horned species inspired the
early painters who decorated the walls of the Lascaux caves in France. It persisted until about
1,200 years ago and ranged from Korea to Spain.
A little ecological history clarifies the swift and widespread decline among all five living
rhino species in more recent times. Before approximately AD 1400, when the Gangetic Plain
first opened to agriculture, greater one-horned rhinos must have been relatively common in
this hot, steamy region. Basing our calculations on densities achieved by rhinos today in
prime floodplain habitat, I estimated that perhaps half a million or more grazed along rivers or
wallowed in oxbows. Around 1900, there may have been 300,000 to 1 million black rhinos in
sub-Saharan Africa. Several hundred years ago, the numbers of white rhinos must have been
equally staggering. And Javan rhinos were so common in colonial Indonesia that they were
considered garden pests and shot by Dutch tea planters. Even Sumatran rhinos then must have
filled Southeast Asian forests with their peculiar whalelike vocalizations. All evidence seems
to point to the conclusion that rarity is a relatively recent phenomenon for rhinoceroses and
that overhunting, then poaching, and habitat loss have been the drivers of their startling de-
cline.
